The present disclosure relates to electronic circuits and methods, and in particular, to switching regulator circuits and methods having reconfigurable inductance.
Switching regulators are circuits that produce regulated voltages or currents by switching passive elements in to and out of different electrical configurations. FIG. 1 illustrates an example of one switching regulator topology. In this example switching regulator, switches 101 and 102 are turned on and off so that an input voltage, Vdd, charges an inductor (L) 103. When switch 101 is closed and switch 102 is open, energy is provided to the inductor 103 from Vdd to generate an inductor current IL. Switch 102 is periodically closed and switch 101 is opened, and the inductor current IL continues to flow to an output, “out”, as energy in the inductor dissipates. Switches 101 and 102 are controlled by a switch control circuit 104, and the ON/OFF time of each switch may vary according to a number of different application requirements. Switching regulators are very efficient circuits for providing voltages and currents, but suffer from a number of problems in certain applications.
For example, one challenge with switching regulators is noise generated by the switches as they are turned ON and OFF. This is referred to as switching noise. The switching noise can be effectively suppressed with the use of a larger inductor. However, using a larger inductor has a drawback of degrading the efficiency, especially if the voltage on the output is changing. It is very difficult to select a single inductor that achieves the best trades off between the noise and efficiency performance over a wide bandwidth range, for example.
One particular application where switching regulators are employed is in envelope tracking in a power amplifier application. Achieving high efficiency in a linear power amplifier is challenging, particularly in wireless applications where modulation schemes have become more complicated and their peak to average power ratio increases. Envelope Tracking (ET) is an approach to boost the efficiency of a PA by continuously adjusting its power supply voltage to improve efficiency during transmission.
FIG. 2 shows an example configuration for one type of envelope tracking system. In this example, an input signal Vin is provided at the input of a power amplifier (PA) 203 to produce a power amplified signal Vout. PA 203 receives a power supply voltage Vdd and a power supply current Idd from a configuration of a linear amplifier 201 and a switching stage 202. The linear and switching stages work together to adjust the level of Vdd based on the envelope of the power amplifier input signal Vin to improve the efficiency of the power amplifier 203. In this example, linear amplifier 201 receives an envelope tracking signals (ET) representing the envelope of Vin, for example. Linear amplifier 201 may produce a voltage Vdd and current Iamp. Switching stage 202 receives a switching signal SW based on the envelope signal. In this example, SW is generated by sensing Iamp. Switching stage 202 produces a voltage Vdd and current Isw. The sum of currents Iamp and Isw are the power supply current Idd drawn by PA 203. The switching regulator stage 202 boosts the ET's efficiency but it is noisy. The linear regulator stage 201 is higher speed and ensures the optimum power supply voltage to achieve PA's peak efficiency, but it is (power) loss. Unfortunately, the noise and efficiency are the contradictory performance requirements.
Envelope tracking effectively improves the efficiency of the PA in the RF transmitter path while inevitably introducing a switching noise, which may fall into the RF receiver band and hence degrades the receiver sensitivity. The frequency content of the switching noise is sometimes coherent with the transmitter band profile, making it difficult to implement a filter to effectively remove the noise. As mentioned above, the switching noise can be effectively suppressed with the use of larger inductor, but with reduced efficiency.
For many existing envelope tracking systems, a choice of the inductor is made to balance the noise and efficiency performance. However, a particular choice of inductor cannot meet the wide range of the envelope bandwidth requirement.